Photonic crystals are periodic dielectric materials which continue to generate considerable interest because of their ability to offer novel ways to control the flow of light, see E. Yablonovitch, Phys. Rev. Lett., 58, 2059 (1987) and S. John, Phys. Rev. Left., 58 2486 (1987). Many of the unusual properties of photonic crystals are based on the existence of a partial or complete photonic band gap, a natural consequence of the material's underlying periodicity. This new class of optical materials provides the basic building blocks for a whole new generation of passive and active optical devices. For example, photonic crystals have found application in high efficiency microlasers (O. Painter et al., Science 284,1819 (1999)), waveguides (E. Chow, S. Y. Lin, J. R. Wendt, S. G. Johnson and J. D. Joannopoulos, Opt. Lett. 26, 286 (2001)), couplers (S. Noda, A. Chutinan and M. Imada, Nature 407, 608 (2000)) and new types of optical fibers (R. F. Cregan et al., Science 285, 1537 (1999)).
The range of devices based on photonic crystals could be enlarged in scope if they could be tuned on very short timescales. Although switching in dielectric stacks (one-dimensional photonic crystals) has been studied for over two decades (see T. G. Brown and B. J. Eggleton, Opt. Lett. 3, 385 (1998)), only a few theoretical studies of switching in higher-dimensional photonic crystals have been reported to date. Such crystals are unique in their ability to control the propagation of light in two or three dimensions, potentially enabling ultrafast integrated optical circuits for optical packet switching and optical computing.
Tuning of 2-D and 3-D photonic crystals has been demonstrated via infiltrated liquid crystals as disclosed in S. W. Leonard et al. Phys. Rev. B, 61, R2389 (2000) and K. Yoshino et al. Jpn. J. Appl. Phys. 38, L961 (1999), but the molecular reorientation responsible for changes in the refractive index typically occurs on a millisecond timescale. Much faster switching can only be achieved using electronic processes. Ultrafast changes in the refractive index can occur via non-resonant processes such as the optical Kerr effect, or resonant processes in which free electrons and holes are created as disclosed in M. I. Gallant and H. M. van Driel, Phys. Rev. B 26, 2133 (1982). The former effect can induce index changes which follow the light pulse, but requires high light intensities. The latter process may be more practical in that it requires substantially lower pump intensity and can still lead to induced changes limited by the pulse width. Relaxation is limited by the carrier recombination time, which can be as short as a picosecond in suitably designed materials, see F. E. Doany, D. Grischkowsky and C. Chi, Appl. Phys. Lett., 50, 469 (1987) and F. W. Smith et al. Appl. Phys. Lett., 54, 890 (1989). Free carriers generated by two-photon absorption have been used to change the optical properties of 1-D Si/SiO system as disclosed in A. Hache and M. Bourgeosi, Appl. Phys. Lett., 77, 4089 (2000), although the induced transmission changes are <0.5% and were measured only at one wavelength.
Recently, P. Halevi and F. Ramos-Mendieta, Phys. Rev. Lett. 85, 1875 (2000) have theoretically shown how thermally activated carriers can tune two-dimensional photonic crystals fabricated with a narrow gap semiconductor (InSb). S. Susa, Jpn. J. Appl. Phys. 39, 6288 (2000) has also theoretically studied how the continuous optical injection of free carriers can shift the band edge of a two-dimensional photonic crystal for moderate pumping intensities. Experimentally, A. Chelnikov et al., Electron. Lett. 34, 1965 (1998) were able to show how free carriers could also control defect mode absorption in a 3-D silicon crystal with a photonic gap in the submillimeter range near 250 GHz.
It would be very advantageous to provide a method for modulating the optical properties of photonic crystals on much shorter time scales for ultrafast control of the propagation of light in photonic crystals. Such a method could be readily adapted to many applications broadly based in those areas of information technology and information handling based on the use of light beams in either all-optical or hybrid (electronic/optical) integrated circuits. As an example, such ultrafast control of light propagation in photonic crystals would enable the production of a large range of dynamic, wavelength tunable photonic crystal-based devices. Particularly, presently available commercial optical switches are based on pure mechanical movements, micro-mechanical movements (MEMS) or optically birefringent materials such as liquid crystals. All three types enable switching times in the microsecond (μs) to millisecond (ms) regions, but do not reach the very short switching times needed as discussed above. To achieve these very short switching times, at present only optical switching can be used which would be achievable using photonic crystal that could be tuned on ultrafast time scales. Another advantage of such a capability would be the ability to perform rapid switching of wavelengths in high-density small form factor optical photonic integrated circuits.